The present invention relates to the filling of emergency cooling towers in nuclear power plants, in particular an improved filling of the type comprising a grid of tubes of plastic material, such as PVC, which is in a position to withstand depressions or overpressures up to 700 Kg/m.sup.2, as required by safety rules.
It is known that the nuclear power plants are required to have an emergency cooling tower, which should be in a position to operate in any environmental condition, even in case of complete destruction of the station, to remove heat from the core where the nuclear reaction takes place, in order to avoid overheating giving rise to temperatures such as to cause the core itself to be melted and possible chain reactions, no longer under control, to result in catastrophic consequences.
In these emergency towers, as well as for the towers adapted to the usual operation the so-called "filling" which provides for the total surface of heat exchange with the water to be cooled, rain-like sprinkling from the tower top, is advantageously comprised of a plurality of planar rows of horizontal parallel tubes, being arranged alternately in a given direction on one plane and in the direction perpendicular thereto on the planes above and below.
It is also known that in the normally operating cooling towers these tubes are fastened at their ends and at given intervals along their length to elongated C-shaped sections, which are perpendicular to the tubes and formed with pluralities of through holes for fitting the tubes therein. The holes are conveniently made by piercing, being arranged in two parallel rows, one above the other, with the holes equally spaced apart and staggered in the two rows. The sections and the tubes fitted therein form, for each filling layer within the tower, a so-called "grid" or hurdle which rests at the ends of the sections on stands or pillars of reinforced concrete perpendicular thereto, which are part of the tower frame structure. Usually the fastening operation merely consists in tying the sections on these stands and, due to the absence of intermediate restraints, the whole hurdle structure is not very resistant, especially to bending stress. On each horizontal plane of the tower filling the grids or hurdles have alternate directions mutually at right angles, with the tubes being directed like the sections of the plane immediately below and vice versa.
This type of filling is not capable of resisting strong stresses, as required to emergency towers, the perfect operation of which is dictated by the safety rules, even under the most unfavourable environmental conditions. This is warranted by the aseismatic concrete structure of the tower in case of earthquake, but the filling structure must be in a position to withstand the strength of a cyclone or a tornado, which for safety reasons has been fixed at a value of overpressure or depression of 700 Kg/m.sup.2. In these conditions, a filling structure as described above can not afford the necessary resistance, since the sections could bend such as to cause the tubes to slip off from the through holes and could break off with the consequent complete destruction of the filling. Therefore filling grids of this type are not suitable for emergency cooling towers.
Then it was also planned to use, instead of C-shaped sections but still adopting low-cost plastic material like PVC, square section support tubes which are much more bending-resistant due to their high moment of inertia. In this case the through holes for passing the tubes will be made in opposite walls of the quadrangular cross-section and directly facing each other, whereby it is no longer possible to make them by piercing. On the contrary other known cutting devices will have to be used and in general the hole diameters will have a lower precision than by piercing, thus resulting in possible, even sensible clearances with respect to the tubes therethrough. Therefore these latter when subject to strong stresses could slide in the holes and slip off from the square-section supports, in spite of the excellent bending-resistance of these.